Method for making anhydrous alkali metal hydrosulfites

ABSTRACT

IN A METHOD FOR MAKING ANHYDROUS, ALKALI METAL HYDROSULFITES BY REACTING SULFUR DIOXIDE AND AN ALKALI METAL FORMATE WITH ALKALI METAL BISULFITES, METABISULFITES AND HYDROXIDES WEREIN AN IMPROVVED YIELD IS EFFECTED BY EMPLOYING AS THE REACTION MEDIUM ALKYL SUBSTITUTED ALIPHATIC AMIDE SOLVENTS, SUCH AS DIMETHYL FORMAMIDE OR DIMETHYL ACETMIDE.

Unitcd States Patent 3,826,818 METHOD FOR MAKING ANHYDROUS ALKALI METAL HYDROSULFITES Joseph Beckwith Heitman, Tacoma, Wash., assignor to Pennwalt Corporation, Philadelphia, Pa. No Drawing. Filed Aug. 3, 1972, Ser. No. 277,706 Int. Cl. C01h 17/66 U.S. Cl. 423-515 4 Claims ABSTRACT OF THE DISCLOSURE In a method for making anhydrous alkali metal hydrosulfites by reacting sulfur dioxide and an alkali metal formate with alkali metal bisulfites, metabisulfites and hydroxides wherein an improved yield is effected by employing as the reaction medium alkyl substituted aliphatic amide solvents, such as dimethyl formamide or dimethyl acetamide.

This invention relates to the making of alkali metal hydrosulfites, and particularly relates to a new and improved method for making anhydrous sodium hydrosulfite, a reducing material Widely used in the bleaching of textiles and of ground wood pulp for newsprint.

Heretofore, the most commonly employed methods for the manufacture of sodium hydrosulfite have been by (1) reduction of sulfurous acid with zinc followed by the metathetical conversion of the zinc salt to sodium hydrosulfite using sodium carbonate or sodium hydroxide, (2) reduction of bisulfite with sodium amalgam and the recovery of the sodium salt from an alcohol solution, (3) electrolytic reduction of sodium bisulfite in a diaphragm cell using graphite or platinum electrodes, and (4) reduction of bisulfite with sodium formate in alcohol solution and recovering the sodium salt from such alcohol solution.

While zinc hydrosulfite has been the major chemical previously employed for bleaching ground-wood, and is widely used for bleaching textiles, zinc salts are objectional from an ecological standpoint by virtue of their toxicity to fish and because of the likelihood of stream contamination and general waterway pollution.

In. general, both the sodium amalgam and the electrolytic reduction of bisulfites have resulted in relatively poor overall yields in the anhydrous hydrosulfite product. Both require the use of dilute aqueous solutions, and there is a natural tendency for the sodium hydrosulfite to decompose readily in the presence of Water or when in the dihydrate state. In addition, the electrolytic reduction procedure requires relatively complex and expensive equipment.

While prior production of sodium hydrosulfite via the sodium formate reduction processes has produced moderate yields (approximately 70 percent based on sulfur and 38 percent based on sodium formate), these earlier procedures utilized an alcohol environment for the reaction medium, principally methyl or ethyl alcohol containing about 20 to 30 percent water. In addition to the need to employ reflux devices on reactors using alcohol because of the latters volatility, the earlier formate reduction methods demanded that the ingredients be added in a precise order for best results and necessitated that the reaction period be protracted for a relatively long time interval, i.e., three hours or more. Furthermore, the alcohol media required carefully controlled conditions of pH, ingredient concentration and temperature. Moreover, the alcohol absorption of sulfur dioxide directly from a sulfur burner is likewise difficult owing to the volatility of the solvent. The sulfur dioxide must be tediously added dropwise to the alkali metal bisulfide (or the alkaline agent, such as sodium hydroxide or sodium carbonate) solution of the alkali metal formate, and reflux equipment is absolutely necessary to maintain the alcohol concentration at the reaction temperature, 60 C. to 70 C.

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Thus, it is well known by way of U.S. Pat. No. 2,010,615, to prepare hydrosulfites through the introduction of gaseous sulfur dioxide into a mixture of sodium formate and sodium bisulfite in aqueous ethanol. However, the product obtained is of relatively poor stability and purity. The reaction of the system may be expressed:

The method of U.S. Pat. No. 3,411,875 which also embodies a sodium formate reduction by introducing sulfur dioxide in methanol into an alkaline agent (sodium hydroxide, -carbonate, -sulfite, etc.) may be expressed by the following reaction:

(1) HCOOM+H O+SO HCOOH+MHSO (2) MOH+SO MHSO where M is an alkali metal Now, I have found that the yield of alkali metal hydrosulfite by way of the formate reduction process can be considerably improved through the employment of certain organic solvents as the reaction medium. This unexpected improvement in hydrosulfite yield is accompanied by an appreciable diminution in reaction time and with greater convenience in operating procedures.

According to the present invention, an alkali metal formate reduction is utilized in a manner superficially similar to either of the foregoing patented processes, but the reaction is performed entirely in a water soluble organic solvent having a mildly active carbonyl, sulfinyl, or phosphoryl group.

The carbonyl, sulfinyl, or phosphoryl group apparently act as a catalyst in promoting the conversion of bisulfites to hydrosulfites. The reason for this catalytic activity is not exactly understood, but the course of the reaction seems to indicate the temporary formation of bisulfite and hydrosulfite addition products.

While it is known that aldehydes and ketones form addition products with bisulfites and hydrosulfites, the activity of the carbonyl groups in unmodified water soluble aldehydes and ketones is too strong to permit their use as solvents in accordance with my invention. Therefore, the solvents which I use either have mildly reactive groups of the above classes or have adjacent :groups which modify or hinder by steric or electronic means the reactivity or catalytic effect of the primary group. For example, although acetone is not suitable as a reaction solvent, dimethylsulfoxide, which is the sulfur analogue of acetone, is sufficiently less active to permit the formation of moderate quantities of sodium hydrosulfite. Better yields, however, result from the use of such materials as dimethylformamide and dimethylacetamide where the activity of the carbonyl group is modified by an adjacent amide group. Tetramethylurea, which has a carbonyl group with two adjacent amide groups, will also exhibit catalytic activity. Likewise, hexamethyl phosphoric triamide (hexamethylphosphoramide) is a useful solvent in this class.

Suitable solvents which may be employed as the reaction medium in the present invention are: dimethyl formamide, dimethylacetamide, N-methylacetamide, die-thylacetamide, dimethyl sulfoxide, tetramethyl urea and hexamethyl phosphoramide.

There is evidence that the reactionsinvolved using the solvent medium of this invention are:

The preferred solvents for the reaction medium are dimethylfor-rnamide (DMF) and dimethylacetamide (DMAC), and I have found that in such a medium a very high quality anhydrous sodium hydrosulfite is produced-a yield of about 85 percent based on sulfur and 69 percent based on sodium formate. My process does not require any special reflux equipment nor is there any care required in the order of adding the reactants. The reaction is quite rapid, reaction time being one-third or less than any of the hitherto practiced commercial processes. The water content of the dimethylformamide reaction medium may be most conveniently in the range of to -35%. The DMF solvent mixture is relatively nonvolatile in comparison to the previously employed aqueous alcoholic environments.

The following examples are illustrative of practice of this invention:

EXAMPLE I 50 grams of sulfur dioxide were first sparged into 500 ml. -('472 grams) of dimethylformamide placed in an 800 ml. beaker. 65 grams of reagent grade sodium formate and 74.5 grams of sodium metabisulfite (equivalent to 81.6 grams sodium bisulfite) were placed in a 1,000 ml. flat bottom distilling flask equipped with a glass tube trombone coil, a thermometer, and a magnetic stirrer. 200 ml. of hot water were added to the flask and the contents heated to 50 C. with stirring. At this stage, the dimethylformamide sulfur dioxide mixture was added from a separatory funnel with stirring of the flask and at such a rate that the addition was complete in about minutes. The temperature elevated to '60 C. during this addition period. The reaction mixture was then stirred for an additional 125 hours during which time the temperature was maintained at 60 C. After completion of this period, the crystals of sodium hydrosulfite were ifiltered off in a Buchner funnel, washed with 50 ml. of ethanol, and dried in a vacuum oven for one hour at 5'5" C.-l-24 grams of product analyzing 93. 1% sodium hydrosulfite were obtained, giving a yield of 84.8% (100% hydrosulfite) based on sulfur.

EXAMPLE II The procedure of 'Example I was followed exactly except that, although the quantity and proportion of sol vent mixture were the same, the ingredients were varied as follows:

Sulfur Dioxide Grams 75 Sodium Metabisulfite do 112 Sodium 'Formate do 95 Ethanol Wash ml 75 192 grams of product analyzing 90.0% sodium hydrosulfite were obtained, giving a yield of 84.5% (100% hydrosulfite) based on sulfur.

EXAMPLE III The procedure of Example I is followed except that 150.5 grams of sulfur dioxide were dissolved in 500 mls. of dimethylformamide for addition to the reaction flask. 47.6 grams of sodium hydroxide and 95 grams of sodium formate were dissolved in 200 mls. of water in the reaction flask. The dimethylformamide solution was added to the flask over a minute period with the flask at 55 C. during the first half of the addition and 60 'C. during the second half. Following the dimethylformamide addition, the flask was stirred for an additional 70 minutes at 63 C. The crystals of sodium hydrosulfite were then filtered off, washed with 75 ml. of ethanol, and dried in a vacuum oven for one hour at 55 C. 182 grams of product analyzing 91.6% sodium hydrosulfite were obtained, providing a yield of 81.7% (100% hydrosulfite) based on sulfur.

Note that the quantity of sulfur used in Example I was the same as in Example II, but was included as sulfur dioxide rather than as a mixture of bisulfite and sulfur dioxide.

As the DMF-S0 mixture is added to the reactor a change in color to a tan or brown is noted. There also may be a blue streak in areas of high concentration of DMF-S0 These color manifestations are perfectly normal and indicative of the presence of S 0 and its polymerization products. However, a bright yellow color in dicates decomposition and the formation of elemental sulfur.

Evolution of carbon dioxide occurs at a point in time when about half of the DMF-S0 has been incorporated. This evolution is initially slow, but increases to a maximum Within several minutes followed by an abatement over the balance of the reaction period. Even at the maximum reaction rate, the carbon dioxide evolution is not violent, with suitable temperature control. The brown color gradually decreases during the reaction and is not evident after the first hour. The solution is colorless and the product is white.

Although it is possible to make sodium hydrosulfite in an almost anhydrous dimethylformamide medium, it has been found that the presence of water results in an improvement in reaction efiiciency. This is believed to stem from the fact that water acts as 'a solvent for inorganic salts and as a reactant for converting disulfur trioxide to dithionic acid, as shown in Equation 5 listed above. The optimum ratio of DMF to water is approximately 7 0-'80% DMF to 30-20% water. On the other hand, water concentrations in excess of 30% in the reactor are likely to cause excessive solubility of product salts in the reaction mixture, thereby reducing yield. 'It has also been found that excess water tends to promote decomposition reactions.

The proportions of reactants to solvent given in the foregoing examples can be varied to considerable extent. it is quite feasible to use sodium metabisulfite, bisulflte, or sulfur dioxide and caustic soda. One may also employ sodium sulfite to balance off the greater amount of sodium ion introduced by an equivalent increase in sulfur dioxide absorbed in DMF. The various ratios of the reactants are not critical and some flexibility is permitted.

It is to be noted that an excess of sodium formate over the theoretical amount is used in the process. Excess sodium formate in the reaction seems to prevent decomposition. Decomposition is characterized by a bright yellow color (not brown or tan). The yellow color is indicative of loss of the product to bisulfites, thiosulfates, and sulfur. Once this decomposition begins it is difiicult to stop and the product should be immediately filtered and dried. Decomposition when it occurs, usually starts toward the end of the reaction period.

Example 11 sets forth the use of 75 grams of sulfur dioxide per 500 ml. of DMF solvent. The reaction can be carried out and equivalent yields can be obtained with a mixture of ingredients such that the above ratio is reduced to 50 grams of sulfur dioxide per 500 ml. of DMF with a corresponding reduction of the other reactants (c.f. Example I).

It is also feasible to raise the concentration of reactants to approximately grams sulfur dioxide per 500 mls. of DMF with a corresponding increase in the amounts of other reactants. However, some solubility problems may be encountered at the higher concentrations because of a tendency to crystallize intermediates on the cooling coil of the reactor vessel.

It is not necessary that the DMF be absolutely anhydrous prior to introduction into the system for absorbing sulfur dioxide. However, it is preferable that the amount of water present be low at this stage since any water introduced into the system is more advantageously directed into the bisulfite-formate end of the process. The absorption of sulfur dioxide into DMF is accomplished extremely readily and the capacity for sulfur dioxide absorption is very high. There is negligible solvent loss. For example, Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd ed. vol. 19, page 420, states that the amount of sulfur dioxide capable of being absorbed inDMlfisz'.

The optimum reaction temperature appears to be approximately 60 C. There is a tendency for the product 'to decompose at temperatures above 65 C. and the rate ofIeactiOn is decreased at lower temperatures. Accordingly, the use of 60 as an optimum temperature permits a couple of degrees swing either way with allowance for saftety and utility. The overall reaction is generally endothermic by calculation, about 4 kilocalories per mole of sodium hydrosulfite. However, inasmuch as the process proceeds in three consecutive reactions, heat is given otf during the first third of the reaction period. Following this first period, heat must be supplied to promote the reactions and boil off carbon dioxide. Therefore a combination cooling-heating coil is provided in the reactor. The bisulfite-formate-water mixture is first placed in the reactor, heated to about 50 C., and the DMF-sulfur dioxide component introduced with moderate stirring provided. While other sequences may be utilized, the yield may suffer somewhat as a result. The DMF addition should be, performed in about to minutes, and during this period heat evolution is the greatest and causing a temperature riseto about 60 C. Cooling is required during this period. Following the addition of the DMF solution, the reactor is stirred moderately for about an additional one and one-quarter hour while the tempermine is maintained at 60 C. or slightly higher.

The crystals formed by the subject process are very 'easilyfiltered although they have a particle size on the 'order of 40 microns (325 mesh). There is no delay in drainage on a Buchner filter and ordinary general purpose filter paper is satisfactory. Since dimethylformamide ha's a low ,vo latility, it is preferable to wash the hydrosulfite product free of this solvent prior to drying. Ap-

proximately 75 mls. of ethanol are used to each 190 grams .of product recovered. Methanol may also be employed. Although there is approximately 30% water in the DMF solution, the sodium hydrosulfite product obtained appears to be in the anhydrous rather than in the dehydrate form. The product is relatively easy to dry and this is best accomplished under vacuum at a temperature in the order of 40 to 60 C.

' The particle size of the sodium hydrosulfite obtained Sulfur Dioxide grams 75 Sodium Metabisulfite do 112 SodiumFormate do 95 Ethanol Wash ml 75 Except for the solvent medium and the formulation, the procedure of Example I was followed. 205 grams of product analyzing 86.7% sodium hydrosulfite were obtained giving a yield of 87.1% (100% hydrosulfite) based on sulfur.

EXAMPLE V The ingredient formulation of Example II was repeated using N-methylacetamide as the reaction medium. 500 mls. of N-methylacetamide were used in conjunction with 200 mls. of water, and the procedure of Example I was again followed.

During the course of the reaction it was observed that considerable thickening of the mixture occurred, but as the reaction proceeded the mixture ultimately thinned out.

87.3 grams of a product analyzing 54.6% sodium hydrosulfite were obtained, giving a yield of 42.8% (100% hydrosulfite) based on sulfur.

EXAMPLE VI The reaction medium was formamide. Again, using the ingredient formulation of Example II and following the procedure of Example I, 500 mls. of formamide was utilized in conjunction with 200 mls. of water.

It was observed that the formamide darkened in color as soon as the sulfur dioxide was introduced, thus indicating an unstable condition.

All of the salts dissolved in the reaction medium and there was no hydrosulfite product yield apparent.

The use of unmodified sulfinyl type solvents is illustrated by the following example using dimethyl sulfoxide. While moderate amounts of sodium hydrosulfite are produced in this case, the yields are not as high as they might be were the reactivity of the sulfinyl group further reduced.

EXAMPLE VII The procedure of Example I was repeated, except that dimethyl sulfoxide was substituted for dimethyl formamide as the solvent reaction medium. An intense inky blue color, indicating the presence of disulfur trioxide was obtained. However, the color disappeared in about one hour leaw'ng a white salt. The sodium hydrosulfite product obtained from the reaction using DMSO as the reaction medium was slimy, and difiicult to filter and dry. The product was also somewhat unstable.

A number of runs were made using DMSO as the reaction solvent combined with various amounts of water. Temperature conditions were varied as well. Moderateto-vigorous stirring was employed in the blender.

The reaction is accelerated by the presence of water, as evidenced by the reduction in intensity of the blue or green color during the initial portion of the reaction when the water-DMSO ratio was increased.

The maximum yield of the hydrosulfite product (based on sulfur) was about 61% and the purity was about 40%, such a yield having been produced when the DMSO-water proportion was approximately :20, and the reaction temperature was in the vicinity of 60 C.

The following example illustrates that tetramethylurea is yet another solvent of the subject category which is useful as the reaction medium for making hydrosulfites. However, solubility problems may arise such that desirable catalytic effects may be offset by the physical conditions during reaction.

EXAMPLE VIII The procedure of Example I was repeated using tetramethyl urea (TMU) as the reaction medium. During the course of the reaction, the mixture became exceedingly thick, although the viscosity decreased as the reaction proceeded.

The yield of sodium hydrosulfite product using TMU as the reaction medium was 55.4 percent hydrosulfite) of a product assaying at 47.9 percent.

tained yielding 39.9% (100% hydrosulfite) based on sulfur. The product obtained utilizing the hexamethyl phosphoric triamide reaction medium was unstable similar to the product made in the dimethyl sulfoxide solvent.

As is apparent from the foregoing description, dimethyl formamide and dimethylacetamide are the preferred reaction media since the reaction rate is slower and more easily controlled. However, the overall reaction rate with the latter two solvents is still three times faster than using either methanol or ethanol. The hydrosulfite product yield with DMF is superior than with alcohol being about 85% and 70% based on sulfur and formate respectively as compared to 70% and 38% respectively when alcohol is used.

Although this invention has been described in considerable detail, such description is intended as being illustrative rather than limiting, since the invention may be variously embodied without departing from the spirit thereof, and the scope of the invention is to be determined as claimed.

What is claimed is:

1. In a method for making alkali metal hydrosulfite by reacting sulfur dioxide and an alkali metal formate with an alkali metal compound selected from the group consisting of alkali metal bisulfites, alkali metal metabisulfites, and alkali metal hydroxides, the improvement which comprises performing the reaction in a medium containing a solvent selected from the group consisting of dimethyl formamide and dimethylacetamide.

2. The method of claim 1 wherein the alkali metal iii the compounds recited is in all instances sodium.

3. The method of claim 2 wherein sodium formate is present in excess.

4. The method of claim 3 wherein some water is present.

References Cited UNITED STATES PATENTS 3,576,598 4/1971 Plentovich et al. 423-515 2,251,216 7/ 1941 Woodhouse 423243 3,538,681 11/1970 Cantrell et al. 423-243 EARL C. THOMAS, Primary Examiner 

